1. Field of the Invention
This invention relates to poly-crystal compound piezoelectric materials possessing piezoelectricity for use in actuators as positioning mechanisms in precision instruments and apparatuses, actuators as driving sources for fluid control valves, and pressure sensors, for example. The invention also relates to methods of manufacturing the piezoelectric materials.
2. Description of the Related Art
Barium titanate (BT: BaTiO3), lead titanate (PT: PbTiO3), lead zirconium titanate (PZT: PbZrO3—PbTiO3) and the like have been reported as piezoelectric ceramics. Among them, PZT has been widely used for actuators, pressure sensors and the like owing to the largest displacement value.
Of those ceramics, BT was found out to be a ferroelectric substance in 1942. Since then, BT turned out to be usable for ceramic polycrystalline, and numerous studies have been therefore pursued to find usage as capacitors, actuators and the like. Meanwhile, there was a discovery in 1955 that PZT possessed an electromechanical stiffness constant more than twice of BT. Since then, PZT has been dominantly used in actuators, buzzers and the like.
However, in recent years, environmental issues on hazardous substances are considered to be more important. Accordingly, needs in developing lead-free piezoelectric materials are increasing. For example, studies on piezoelectric materials using a Bi0.5Na0.5TiO3 (BNT) compound discovered in 1961 (Smolensky et al., Soviet Physics Solid State [2] 2651-54, (1961)) are now in progress.
Japanese Patent Publication Sho 62-202576 (published in 1987) disclosed piezoelectric ceramic using a BNT-MTiO3 (M; Ba, K0.5Bi0.5) compound. However, this ceramic has a defect of generating lateral vibration interference and expanding vibration when used in an ultrasonic flaw detector or a thickness gauge, because a stiffness constant Kp in a radial direction of BNT-MTiO3 is greater than a stiffness constant Kt in a thickness direction thereof.
Similarly, BNT-MTiO3 (M; Sr, Ca, Pb) was reported in a thesis by T. Takenaka et al. (Ferroelectrics, Vol.106, pp.375-380, (1990)). However, a piezoelectric constant d33 of BNT-MTiO3 reported therein is about 120 pC/N, which is about ¼ of PZT.
Moreover, characteristics as an electro-optic crystal were reported concerning Sr1−xBaxNb2O6 (SBN) formed into a single crystal in a range of x=0.5 to 0.7 (S. Sakamoto and T. Yazaki, Appl. Phys. Letters Vol.22, p.429, (1973)). Since then, SBN has been used for infrared detectors and surface acoustic wave filters. Meanwhile, piezoelectric characteristics have been reported on a single crystal of Sr2−xCaxNaNb5O15 by R. R. Neurgaonkar et al. (Ferroelectrics, Vol.160, pp.265-276, (1994)). Furthermore, Ba2−xSrxNaNb5O15 has been developed as a material for filters (Japanese Patent Publication H10-297969 (published in 1998)).